Raceway ponds are one of the simplest systems for mass cultivation of algae and have been used since the 1950's. In such a system, fresh culture medium is fed into an open pond where it cultured in natural environmental conditions. The pond is typically about 15-20 cm deep and is designed in a raceway configuration (a closed loop re-circulating channel). Pumps or a paddlewheel can circulate the algal cells and nutrients while baffles in the channel guide the flow around the bends in order to minimize space. The main disadvantage of an open system is the environment to which the pond and its contents are exposed. The pond is susceptible to evaporation, contamination by organic and inorganic sources, including unwanted biomaterial and proves to be problematic in maintaining optimum culture conditions. The open pond configuration also reduces the efficiency of microalgae use of carbon dioxide, thus limiting biomass production. Using sunlight as the energy source and being open to the environment, a limitation on the growing season and a diurnal and seasonal variability in the production rate may also be encountered depending on the geographic location.
Many different designs of enclosed photobioreactors (PBR) have been developed to overcome problems associated the open ponds, and some of the deficiencies have been mentioned above. Tubular reactors use ambient sunlight or can utilize an artificial light source as the energy source and are often considered for commercial large scale application. The reactors utilize arrays of long transparent tubes; however, the tube length is limited by pH variation, O2 accumulation and CO2 depletion. The tubes can be organized in a several configurations ranging from horizontal to helix.
Bubble column and airlift reactors are examples of vertical tubular reactors. Air is bubbled into the reactor at a bottom end to provide good mixing and sufficient CO2 feedstock, while adequately removing O2. This design offers an efficient gas transfer rate, but a major drawback of this reactor type is the angle of sunlight relative to the orientation of the tube; a large fraction of the incident light is reflected and not directed to the biomass being grown. The solar orientation with respect to the reactor changes diurnally and seasonally. Another drawback of this configuration is that the diameter of the tubes needs to be relatively large to provide high culture volume and efficient gas transfer. This decreases the area-to-volume ratio which decreases light utilization, thus decreasing the reactor efficiency compared with that of a flat panel reactor.
A reactor with a horizontal tube configuration allows more efficient light utilization than the vertical tubular reactor and unlike the vertical reactor typically has a gas transfer system. A drawback to this configuration is the orientation of the tubes. Although light harvesting increases, plot area required also increases, as well as the expense for the large number of tubes required to attain industrial scale volumes (5,000-10,000 L).
A helical tubular reactor is often configured with tubes coiled in an open circular framework. A centrifugal pump is used to circulate the culture thought the tubes. Although the technique has high light utilization, it is not as efficient as the flat panel. This is due to the orientation of the tubing relative to the sun; a fraction of the incident light is reflected and not directed to the biomass. Another drawback of this particular reactor is the use of a centrifugal pump; this can cause shear stress and hence cell damage.
Flat panel reactors are typically constructed with shallow depth panels to attain a high area-to-volume ratio. This design reduces the light path and increases light utilization efficiency. Algae cultures in these reactors are typically 2-4 cm thick with light being absorbed at the top of the culture within the first few millimeters, so only a fraction of the culture is being directly exposed to the incident energy. Systems have been constructed and tested using alveolar panels. The main disadvantage of this is oxygen buildup due to the small diameter of the reactor.
TABLE 1Summary of the salient features of biomass production systemdesign types.ReactorDesignTypeAdvantageDisadvantageRacewaySimple designEvaporation and contaminationPondsFair to good mixingDifficult species controlLow biomass production comparedwith photobioreactor [Poor gas transferNo temperature controlVerticalEasy cultivation ofDifficult scale up. Dark zones aretubularmicroalgaeproblematic in scale up.reactorMost efficient gasLess efficient than flat panel reactortransfer ratein light collectionExcellent temperaturecontrolHorizontalExcellent light transferOrientation of tubes requires moretubularExcellent temperaturelands space than a vertical reactor.reactorcontrolLess efficient gas transfer rate than aEasy species controlvertical reactorHelixSmall land area forLess efficient than flat panel for lighttubularlarge volumescollection.reactorUniform mixingPumping to drive the culture to theExcellent lighttop of the tubing can cause cellutilizationdamage.Excellent temperaturecontrolFlat panelSupports the highestOxygen build upreactordensity of cultureDifficult scale up(high productivity).Excellent lightutilizationExcellent temperaturecontrolHigh gas transfer rateUniform mixing
Open systems such as raceway ponds, have the advantage of low capital cost, but photobioreactors offer better control of growth conditions, superior contamination control, and better containment of genetically modified organisms. In addition, photobioreactors may be used year-round, even in colder climates